Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with Magnetic Tunnel Junctions (MTJ), is a major emerging technology (1, 2), highly competitive with existing semiconductor memories (SRAM, DRAM, Flash etc). The key element of MRAM technology is the MTJ element. The MTJ consists of two ferromagnetic layers (free layer and pinned layer) separated by a thin tunnel barrier layer. Magnetization of the two ferromagnetic layers can be arranged in either parallel (low resistance) or anti-parallel (high resistance) magnetization states to, respectively, represent “1” and “0” memory states. In MRAM array cells, information is stored in the MTJ's free layer.
The MTJ memory cells are inserted at the back end of a CMOS process. The high-speed version of MRAM architecture consists of a cell with an access transistor and a MTJ (1T1MTJ) applying currents to orthogonal conductor lines. The conductors are arranged in a cross-point architecture that provides the field needed for selectively switching each bit. The intersection of the lines generates a peak field that is engineered to be just over the switching threshold of that MTJ.
The generic MTJ structure is schematically illustrated in FIG. 1. Seen there is bottom conductor 21, seed layer 22, AFM pinning layer 23, pinned ferromagnetic layer 24, tunneling barrier layer 25, free ferromagnetic layer 26, and capping layer 27. The three most critical layers in the MTJ stack are (a) pinned layer, (b) tunneling barrier layer, and (c) free layer.
In most MTJ devices the pinned layer is a synthetic antiferromagnetic trilayer (e.g. CoFe/Ru/CoFe) that serves to reduce the offset field applied to the free layer. The tunneling barrier layer most widely used at present is an aluminum oxide (AlOx) layer formed by first depositing a 7-12 Å thick Al film, which is subsequently oxidized, in-situ, by various means. The MTJ's free layer is best made of a thin permalloy (NiFe) film, selected for its reproducible and reliable switching characteristics—low switching field (Hc) and good switching field uniformity (σHc). The intrinsic dR/R that is obtainable for a NiFe-MTJ is, at best, around 40% for a R.A value (resistance.area product) between 1,000 to 10,000 ohm-μm2.
We note here that present 1 Mbit MRAM chips are designed as arrays of 0.3×0.6 μm2 bit size MTJ elements that are capable of delivering dR/R=40% and RA =1000-2000 ohm-μm2. The MTJ during a read operation is biased at 300-400 mv. At this bias voltage, the effective dR/R is around 25%.
For even higher density MRAM chips (e.g. 250 Mbits), MTJ bit size would be reduced to less than 0.2×0.4 μm2. For the next generation MRAM, it is required to have MTJ elements capable of delivering much higher dR/R (>>40%) with lower MTJ resistance (e.g. R.A=500 ohm-μm2) to improve read access time (3).
It has been shown that MTJs made with a monocrystalline MgO barrier layer and a CoFe(B) free layer are capable of a very high dR/R of more than 200% (4-6). Such a huge dR/R is the result of coherent tunneling (7) in which the electron symmetry of the ferromagnetic electrode is preserved during tunneling through the crystalline MgO barrier. In reference (5), strongly 001 oriented MgO is formed on top of the crystalline oriented CoFe pinned layer (AP1).
The MgO was formed by reactive sputtering of a Mg target in a Ar/O2 gas mixture R.A for these MgO MTJs was greater than 10,000 ohm-μm2. The MgO-MTJ described in reference (6) is made in the Anelva C-7100 sputtering system. Highly oriented (001) MgO is also formed on top of an amorphous CoFeB pinned layer (AP1), a R.A of 460 ohm-μm2 being reported. Some typical data relating to structures of this type* are summarized in TABLE I below:
TABLE IMagnetic performance of an MgO-MTJ formed in an Anelva C-7 100sputtering system.FLCappingR.AMRBsHcHinHkCoFeB30Ta80/Ru10017092140.339.931.7634.6NiFe40Ru30/Ta30/Ru100213043.50.291.634.59.5CoFeB30Ta80/Ru1007442190.258.21−0.7738NiFe35Ru30/Ta30/Ru100122846.00.241.704.148.8FL = free layer; MR in %, Hk, Hin, and Hc in Oe;
The specific structure was:    Ta50/CuN200/Ta30/MP150/CoFe(30%)25/Ru8.5/CoFeB30/MgO18/FL/Capping (360° C.-2hrs−10K Oe).
It is important to note that in this case of a CoFeB/MgO/NiFe MTJ, the crystalline MgO tunnel barrier, formed by RF-sputtering from an MgO target, does not match well with the NiFe free layer.